The present invention relates to the field of gas springs, and specifically to a soft-start, soft-return gas spring for, among other things, cushioning the action of draw dies operating in stamping presses and the like.
For many years, double action or xe2x80x9ctogglexe2x80x9d presses were the industry standard for forming large metal parts such as automobile hoods. A toggle press has an outer ram that comes down and binds the blank to be formed. An inner ram with a punch having the desired part shape then follows through to draw the blank into a complementary shaped die cavity. In the quest for speed and efficiency, much of the industry is now using straightside or transfer presses which is the forming press to form the initial shape from the flat metal blank. Next, the part passes through a series of individual stations or presses to complete the necessary die operations, all in one combination process. Unfortunately, toggle presses are relatively slow and form the part in an inverted or upside down orientation. In most cases then, the toggle press will most likely have to include a turnover station following the draw operation. A solution to the speed and inversion problem is the use of the straightside press. Unlike the toggle press, where the outer ram comes down gently to bind the blank for drawing, straightside presses have but a single ram with an upper platen that is actuated by the throw of the press crank cycling at up to 30 strokes per minute and 30 or more inches of stroke. With a draw die mounted in the press, the die cushion or lower binder surrounds a lower punch which defines the complementary part shape to the cavity of the upper die. The cushion floats around the punch and is supported in an up position upon a series of nitrogen gas springs that collectively offer adequate force to bind the blank for the draw operation. When the upper die binder face meets the floating cushion and blank, the blank is instantly contained between the upper and lower binder faces. The impact from the upper binder meeting the stationary die cushion is violent. The shock caused by this impact causes great damage to the press drive and creates undesirable pressure spikes in the individual cushion unit seals. After contact, because the ram force exceeds the resistance force of the gas cylinders, the ram, blank and cushion continue downward at the automatic press cycle speed until the they reach the bottom of the stroke, at which point the blank has been formed to the desired shape. At this point, the cushion cylinders have been compressed, and their resistive force has increased in accordance with the compression ratio of the nitrogen gas (Boyle""s law). Cushion forces for major automotive dies commonly operate in the range of 200 to 300 tons. When the press ram reaches bottom position and starts its upstroke, the nitrogen gas cushion springs with their intensified pressure forces against the upper die throughout the die cushion upstroke. These forces cause major press drive damage, and stamping facilities have long been seeking a method to greatly reduce the cushion forces at the bottom of the press stroke so this intensified cushion force does not follow through causing such damage. Thus, while gains have been made in speed and efficiency from the use of straightside presses versus toggle presses, the wear and tear inherent in the application of straightside presses continues to plague its users.
What is needed is a way to abate or eliminate the wear and tear resulting from the high impact and recoil effect inherent in straightside presses using nitrogen spring-loaded die cushions and to reduce the die cushion xe2x80x9cupxe2x80x9d force such that the primary cushion force will not follow the press ram xe2x80x9cupxe2x80x9d stroke.
Generally speaking, apparatus is provided for cushioning the action of draw dies operating in a straightside stamping press. The apparatus includes a soft-start, soft-return gas spring that provides a die cushion with desired high tonnage resistance to movement when the press ram slams the die binders together, thereby enabling proper binding of the blank, but conversely permits the ram to return to its upper position with a greatly reduced cushion resistance throughout the full upstroke.
A soft-start, soft-return gas spring includes an outer cylindrical tube; a base plate disposed at the bottom of the tube; an annular-shaped head plate disposed at the top of the tube and having an inner cylindrically-shaped wall; an inner tube mounted to extend between the head plate and the base plate coaxially within the outer tube; a piston/rod assembly having a main piston and a rod and being coaxially mounted in the inner tube to reciprocate vertically therein between a retracted, compressed position and an extended, rest position, the rest position including at least a portion of the rod extending through the inner cylindrically-shaped wall of the head plate and upwardly of the head plate a first distance and the main piston disposed up near the head plate; an annular-shaped valve ring coaxially mounted around the rod to reciprocate between the main piston and head plate; wherein the outer tube, base plate, head plate, inner tube, and main piston define a primary gas chamber; wherein the inner tube, main piston, piston rod, valve ring and head plate define a relief chamber; seals for preventing fluid flow from the primary and relief chambers; a passageway providing communication between the primary gas chamber and relief chamber; and, valve apparatus, including the valve ring, for controlling the fluid flow through the passageway between the primary and relief chambers. The valve apparatus blocks the fluid flow from the primary to the relief chamber following a very short downstroke, soon after a load is applied to the piston rod to move the piston/rod assembly from the extended, rest position toward the retracted, compressed position. And from the retracted, compressed position, once the load is removed from the piston/rod assembly and piston/rod assembly begins to move back toward the extended, rest position under the force of the gas pressure in the primary gas chamber, the valve apparatus begins to open the passageway to allow fluid to flow from the primary chamber to the relief chamber, thus equalizing the gas pressure in the primary and relief chambers and significantly reducing the pressure biasing the piston/rod assembly back to the extended, rest position.
It is an object of the present invention to provide an improved gas spring.
Further objects and advantages will become apparent from the following description of the preferred embodiment.